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Tuesday, June 6, 2017

A Small Basal Paravian Not Preserved as Roadkill? Enter Liaoningvenator!

Near the base of Paraves where we once had only Archaeopteryx as a model for the last common ancestor of modern birds and dromaeosaurids, we now know of a plethora of protobirds that likely occupy the same general region of theropod phylogeny. The Late Jurassic Tiaojishan Formation in northeastern China has proven particularly productive in this regard, its fine-grained lake deposits playing host to Pedopenna, Anchiornis, Xiaotingia, Eosinopteryx, and Aurornis, with rumors of more to come.

Many of these early paravians are known from essentially complete remains, have their soft tissues preserved in exquisite detail, and, in the case of Anchiornis, are represented by hundreds of specimens, to the point where their integument, musculature, and even coloration can be restored with unprecedented accuracy. On the flip side, however, these specimens tend to be preserved as flattened corpses similar to roadkill, making aspects of their skeletal anatomy challenging to interpret.

Another geological formation exposed in northeastern China, the Early Cretaceous Yixian Formation, is also known for lake deposits containing "roadkill" fossil specimens with finely-preserved soft tissues. However, the Yixian has other fossil beds that were formed by volcanic ash, and though the fossils found in these generally lack obvious soft tissue structures, they can preserve complete skeletons in three dimensions. A newly-named basal paravian has been recovered from these Yixian ash beds, making it a potentially quite significant find.

The holotype of Liaoningvenator, from Shen et al. (2017).

Liaoningvenator curriei is known from a nearly complete specimen, and at a glance it has some striking features. It looks extremely leggy, with very long lower legs and and feet for its size. Based on rough measurements done in ImageJ, the longest metatarsal is around 64% the length of the tibia, comparable to the ratio seen in some cusorial noasaurids such as Elaphrosaurus. Its forelimbs, conversely, are quite short, the humerus being less than 60% the length of the femur. (Compare Jinfengopteryx, another short-armed basal paravian, in which this figure is around 70%.)

The tail also looks unusually short and the life restoration provided in the paper appears to take this at face value, depicting the preserved length of the tail as its total length in life. The tail as preserved looks truncated, at least to my eye, so I, for one, am skeptical of this interpretation. (The authors do not comment on this issue one way or another.) Even so, it may not be farfetched to suggest that even the complete tail of Liaoningvenator was fairly short, considering that the phylogenetic analysis in the description finds it to be the sister taxon of Eosinopteryx, which has a relatively short tail with only 20 tail vertebrae in total. (For comparison, Anchiornis and Aurornis both have around 30 tail vertebrae.)

Speaking of phylogenetic affinities, the description recovers Liaoningvenator and Eosinopteryx as part of a clade of basal troodontids along with Anchiornis and Xiaotingia. If these findings are valid, Liaoningvenator would be the geologically youngest known member of this basal clade. It would also be the largest member of the group by far: using the Christiansen and Fariña (2004) method of estimating theropod body mass by femur length, Liaoningvenator is predicted to have weighed around 2 kg, whereas its Tiaojishan brethren have all been estimated as less than 1 kg. (On top of that, bone histology indicates that the holotype of Liaoningvenator was still growing at the time of death, though approaching skeletal maturity.)

That's all very fascinating if true. However, the phylogenetic relationships of these basal paravians are notoriously difficult to figure out. Anchiornis (the best-studied Tiaojishan paravian) has bounced between being an avialan, a troodontid, and neither ever since its initial description. In addition, it is not clear whether all of these protobirds really do clade together to the exclusion of other paravians. Perhaps we should also expect a tumultuous phylogenetic future for Liaoningvenator.

I would be remiss if I neglected to mention that Liaoningvenator is not the only small basal paravian known from a complete, three-dimensionally preserved specimen. Mei is known from two suchspecimens, also found in ash beds from the Yixian Formation. Infamously, both of these specimens have been preserved in what appears to be a sleeping posture, which has led to jokes that Mei spent all of its time sleeping, despite efforts by paleoartists to depict alternative behaviors. Unlike Liaoningvenator, however, Mei has never been considered a close relative of the Tiaojishan "problem paravians", and is uniformly recovered as a troodontid.

3 comments:

Yeah, I noticed the same sort of issue for the tail in the holotype when reconstructing the skeletal. I was generous and added a few more, giving it 20 or 21 (can't recall. Pretty certain 21). And I can honestly only guess what this animal was doing with those limbs. Kicking, wading, etc, only time knows. Great post, keep it up.